The present invention relates to electrodeless fluorescent lamps, and particularly to high light output closed-loop lamp utilizing the transformer coupling of the primary winding to the plasma current.
The concept of a closed-loop electrodeless lamp was described by Anderson in U.S. Pat. No. 3,500,118. The lamp comprised a closed-loop tubular envelope, one or more ferrite cores encircling the tube and one or more turns wound on the core. The alternating voltage applied to the primary winding, Vc, induces an alternating magnetic field, Bc, in the core that in turn generates a closed loop alternating electric field, Eind in the envelope. The electric field, Eind, generates in the envelope an inductively coupled discharge with the xe2x80x9cclosed-loopxe2x80x9d current Idis, that works as the secondary winding.
In U.S. Pat. No. 3,500,118, the lamp tube had a diameter about 3 cm and a path length of about 88 cm. The buffer gas, argon, was at the pressure between 1 and 5 torr. The lamp was operated at frequency, f greater than 50 kHz, at power, Plamp=30-40 W, and with the discharge current, Idis=0.2-0.5 A. At such discharge conditions, the power losses, Ploss, in ferrite cores were relatively high (10-15 W) that resulted in low lamp power efficiency, xcex7=Ppl/Plamp less than 80%.
A substantial improvement of the power efficiency and luminous efficacy of a closed-loop lamp was achieved by Godyak et al., U.S. Pat. No. 5,834,905. By increasing the discharge current, Idis, from about 0.5 A to about 5 A and lamp power from about 40 W to about 150 W, Godyak et al. substantially reduced the ferrite core power losses, Ploss, from 11-18 W to 3-7 W. As a result, the power efficiency of the lamp operated at 150 W, was increased to 95% and the luminous efficacy was increased to 94 LPW. Both U.S. Pat. Nos. 5,834,905 and 3,500,118 employed the same coupling arrangement, an induction coil with several turns being wrapped around a ferrite core.
The ignition of an inductive discharge in the closed-loop envelope has to be preceded by the ignition of a capacitive discharge that required a high voltage of few hundred volts. The lamp described in U.S. Pat. No. 3,500,118 employed an additional coil and starting circuitry. In U.S. Pat. No. 5,834,905 patentees used a special xe2x80x9cstarting stripxe2x80x9d attached to the envelope surface and starting circuitry.
In neither of U.S. Pat. Nos. 3,500,118 and 5,834,905, did patentees specify the type of wire used in coils nor count the power losses in the coil wire. In cases when the coil has a large number of turns, the wire resistance can constitute a substantial part of the total coil/core resistance and can contribute to the total coil/core power losses.
The lamp built in accordance with U.S. Pat. No. 5,834,905 employs inert buffer gases (Ar and Kr) at pressures below 500 mTorr, in particular, at 200 mTorr. This is supposedly optimum gas pressure for a tube having a diameter of 5 to 6 cm. While for lamps with smaller tube diameter, 2 to 4 cm, the optimum buffer gas pressure is expected to be higher than 500 mTorr.
An electrodeless lamp of the closed-loop shape operated at frequencies of 200-600 kHz and lamp power of 100-250 W but without a ferrite core was described in U.S. patent application of Popov, Ser. No. 09/256,137, filed Feb. 24, 1999, now U.S. Pat. No. 6,288,490 and owned by the same assignee as the present invention. The closed-loop, inductively coupled discharge was generated inside the envelope by the electric field, Eind, induced in the envelope with the help of an induction coil only. No ferrite core was used. The coil was made from several turns positioned along the envelope walls, inside the closed-loop formed by the envelope. All turns were parallel to each other and to the closed-loop axis resulting in an axially uniform plasma and visible and UV radiations.
The coil of 10-14 turns consumed no more than 10 W at the lamp power of 150 W that resulted in high lamp power efficiency of 93%, and high luminous efficacy of 85 LPW. However, when the lamp was operated at a frequency, f less than 200 kHz, the power losses in the coil were high,  greater than 20 W.
According to the present invention a novel arrangement is disclosed that results in an efficient closed-loop electrodeless florescent lamp employing an induction coil and ferrite cores and has high power efficiency and luminous efficacy comparable to those described in U.S. Pat. No. 5,834,905 and U.S. patent application Ser. No. 09/256,137, now U.S. Pat. No. 6,288,490.
The invention involves an electrodeless fluorescent lamp formed from straight glass tubes each having approximately the same diameter. All tubes are sealed to each other, thereby forming a closed-loop envelope. The closed-loop envelope can also be made from a single tube bent into a circle or ellipse so two opposite tube""s openings are sealed to each other. A protective coating is deposited on the vacuum side of the envelope and the phosphor coating is deposited on the protective coating.
A filling inert gas at the pressure of higher than 500 mTorr (argon, krypton or the like) and at least one of the vaporous metal such as mercury, are placed in the envelope.
A high frequency induction coil comprises of one or more turns up to 50 each having approximately the same diameter and length and disposed on the atmospheric side of the envelope""s walls thereby forming the closed-loop. The loop is made from multiple strand wire (Litz wire). The number of strands vary from 40 to 600. The number of turns vary from 1 to 20. The gauge number can vary from #30 to 46. One or more ferrite cores up to four are disposed on the envelope so to encircle the tube and the adjacent segments of the coil""s turns.
A high frequency (HF), 50-1000 kHz, power source is coupled to the induction coil via a matching network to ignite and maintain a HF discharge in the envelope. The closed-loop HF discharge generates an axially uniform plasma which in turn produces axially uniform visible and UV radiations.
An object of the present invention is to design an effective closed-loop electrodeless fluorescent lamp operating at frequencies 50-1000 kHz and HF power from 10 W to 5000 W.
Another object of the present invention is to design an assembly of the induction coil and ferrite core that has high core/coil inductance to operate at low frequency as low as 50 kHz.
Yet another object of the present invention is to design a coil that can start a capacitive discharge in the envelope without employing a special circuitry.
A further object of the present invention is to design a lamp where the buffer gas and mercury vapor pressures in the envelope were sufficient to provide high lamp luminous efficacy.